In the recent past, Signode Corporation, the assignee of the entire interest of the present invention, has developed several machines for feeding strap in a chute to form a loop around an article to be strapped and for tensioning the loop tight about the article. Typically, these machines also apply a seal to the tensioned loop or otherwise form a connection between the overlapping strap segments, and then sever the tensioned and sealed loop from the standing, or trailing, length of strap.
There are a number of ways to join the overlapping lengths of strap together. With metal strap, independent crimp-type seals may be applied around the overlapping lengths of strap or slits may be cut into the overlapping lengths of strap so that they interlock, under tension, and prevent the straps from separating. With plastic strap, the overlapping lengths of strap may be fused together by applying heat, either through the introduction of a separate heated member or as a result of friction that occurs when one of the lengths of strap is rapidly vibrated in contact with, and relative to, the adjacent length of strap.
Some types of strapping machines first form the joint with tension existing in the standing portion of the strap which trails the joint area and then subsequently sever the trailing length of strap. Other types of machines, while holding the overlapping lengths of strap together, first sever the trailing portion of the strap while it is still under tension and then form a joint in the overlapping strap lengths. In either case, the failure to relieve all tension in the trailing portion of the strap length prior to a subsequent step has certain deleterious effects. For example, in those machines where a joint is first formed and then the trailing portion of the strap is severed while under tension, the unbalanced reaction force, exerted upon the joint owing to sudden release of tension in the trailing portion of the strap upon severance, creates a sudden shock loading upon the joint which tends to loosen or otherwise degrade the integrity of the joint. In those machines that first sever the trailing portion of the strap while the trailing portion is still under some amount of tension, a similar impact loading results which can jar the mechanisms holding the overlapping strap lengths together which may then cause one to slide relative to the other under the influence of the tension existing in the loop. The effect is to reduce the final applied tension in the loop.
Equally as important, the impact loading on the machine per se resulting from the sudden release of tension upon severance of the strap is undesirable from the standpoint of machine component wear. This is important because the strapping machines are cycled as each article is strapped and thus, shock loading on the machine components is repeated with every article strapped.
With relatively low levels of loop tension, shock loading on the formed joints or the machine components is usually not a serious problem. However, with the advent of increasingly high loop tension levels, such shock loads are becoming more significant. In particular, high shock loads can cause the joints to become loosened or weakened. Thus, it would be desirable to provide a method and apparatus for forming such an interlocking slit-type joint in metal strap wherein the joint and machine are not subjected to impact or shock loading upon release of the tension in the trailing portion of the strap. With some methods in use today, the joint is formed in an untensioned section of the loop and a loss of tension is experienced when the joint is subsequently subjected to full tension. It would be desirable to form a joint under tension so that little or no loss of tension occurs after the joint formation is completed.
Although machines exist for automatically strapping a package with metal strap and for forming a joint in the loop with an independent notch-type or crimp-type seal, and although machines exist for applying an interlocking slit-type joint to a loop of metal strap about a package, the applicant knows of no fully automatic machine which (1) encircles a package with a tensioned metal strap loop, (2) holds tension in the loop by pressing a punch member against the overlapped strap joint area, (3) with the same punch, forms an interlocking slit-type joint in the loop, while at least one of the overlapped strap lengths in the joint region is under tension, and (4) severs the trailing portion of the strap therefrom without subjecting the overlapping strap portions, the formed joint, or the machine per se to shock loading owing to sudden release of tension in the trailing length of strap. It would be desirable to provide a method and apparatus which could be used for strapping articles in such a manner as to form a joint with at least one of the strap lengths under tension and still avoid imposition of shock loading. Further, it would be desirable if such a method and apparatus were specifically adapted for use with metal strap and the formation of an interlocking slit-type joint therein.
With automatic strapping machines wherein the strap is first encircled about the package and sealed with an independent crimp-type seal or, if plastic strap is being used, sealed by fusion, it has been necessary to provide a separate anvil member or strap gate between the package and the sealing unit to provide a sufficient bearing surface against which the overlapping strap lengths are pressed in response to the action of the sealing or joint forming member. The use of a separate anvil or strap guide gate between the package and the overlapping strap lengths thus introduces some amount of slack into the tensioned loop. When this anvil or gate is subsequently removed from between the package and the strap after the joint has been formed, the residual loop tension is decreased. Thus, it would be desirable to provide an apparatus for forming a joint in overlapping lengths of strap wherein a separate anvil or gate can be eliminated.
In strapping machines in use today, a number of methods are employed to terminate the strap feeding process once a complete loop has been formed with the strap and after the strap leading end segment, or free end, has overlapped a portion of the loop. However, each of these methods has certain drawbacks.
In one method, a predetermined length of strap is fed by accurately controlling the feeding cycle of the machine. This involves accurately indexing the strap feed, or traction, wheel a certain number of rotations. Such a method requires complicated and expensive motors and control systems.
A second method requires feeding of the strap at a constant feed rate for a predetermined length of time. Such a method involves a timer control circuit and is inherently less reliable than the other methods that directly control the length of strap that is fed.
Another method for terminating the strap feeding process upon formation of the loop is to provide a motor cutoff limit switch actuated by a sensing lever in the strap chute, the lever being impinged by the strap free end after formation of a complete loop. Machines that employ a strap feed termination system with a limit switch and sensing lever typically have the lever located "ahead" of the strap sealing unit. Such machines rely upon motor momentum to feed the strap free end beyond the sensing lever and into proper alignment with the sealer unit.
Regardless of which method is employed, there is a tendency, after the strap leading end has come to rest, for the strap loop to flex inwardly under its own elasticity and pop out of engagement with the strap chute (especially at "corners" of non-circular chutes). This reduces the size of the loop slightly and effectively forces the strap leading end to slide beyond its original stopping point. The amount of such "self-feeding" can be as much as four inches in typical strapping machines and may produce an undesirable amount of overlap. Since the amount of "self-feeding" is dependent, at any instant, on the precise force with which the strap is fed into the chute by the feed wheel, upon the amount of dirt buildup in the chute, etc., the amount of "self-feeding" cannot be easily predicted and controlled from cycle to cycle. Thus, it would be desirable to provide a method and apparatus for gripping the strap leading end upon the termination of the power feeding step to prevent subsequent strap self-feeding and excessive overlap.